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S-20R circuit "analyisis" difficulties

don Root All Messages By This Member #31942 开云体育 An aside from ongoing topic ??S-20R mixer cct. ?… While trying to “see” how the LO circuit works? things got me in a big knot. C38 seems to run from some osc tanks back to the mixer RF tank area. When I follow that I get to? the common [not ground] of the coils/transformers RF tanks? and found them to be floating = strange to me. I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] ? IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting” ? -- don??? va3drl
Allthumbs All Messages By This Member #31943 Hi all, ? Attached is a pdf of a crystal-clear re-drafting of a S-20R schematic. Perhaps it will be of use to anyone working on one of these sets. ? All credit to the author whose username on the Antique Radio Forum is bcascisa ? Tom toggle quoted message Show quoted text On 03/06/2025 3:36 PM PST don Root <drootofallevil@...> wrote: ? ? An aside from ongoing topic ??S-20R mixer cct. ?… While trying to “see” how the LO circuit works? things got me in a big knot. C38 seems to run from some osc tanks back to the mixer RF tank area. When I follow that I get to? the common [not ground] of the coils/transformers RF tanks? and found them to be floating = strange to me. I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] ? IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting” ? -- don??? va3drl S20-R SCHEMATIC.pdf S20-R SCHEMATIC.pdf
Richard Knoppow All Messages By This Member #31944 Not sure I understand your question so feel free to disregard this if its nonsense. T-10, 11, 12, 13 are the plate tank of the RF amplifier stage. The common connections go to B+ the other side go to one side of the the band switch. The secondary coils go to the other side of the bandswitch which then goes to the signal grid of the mixer tube. The LO is sort of a separate tube within the mixer. It has a common cathode with the mixer part and a common grid. The plate is separate. The LO coils are T-14, 15, 16 and L-1. These coils/transformers go between two sections of the band switch which connects one side to the oscillator grid and the other to the oscillator plate. I have lost track of whether the set does not work on any band or if its the low band only that isn't working. About telling if the LO is working, I agree that the simplest method is to use another radio receiver to pick up the LO. It should be audible without direct connection between the receivers, or just a wire between the antenna terminals. I did try it with a Millen GDO without success, I used the GDO as a hetrodyne detector (most sensitive arrangement) but could not hear the LO in the headphones. However, I coupled to a stub antenna connected to the receiver rather than trying to couple to the oscillator coils. I suspect that coupling to the oscillator coils might load the oscillator enough to kill it. In any case another receiver will work. Keep in mind that the LO will be 455Khz ABOVE the signal frequency on all but the highest frequency band, where it will be 455 Khz BELOW the signal. Also, I did my GDO test on an S-40A, which is very similar but uses a different converter tube so it might not be valid. There is a data sheet on the 6K8 at Franks Tubes on the web and a fair amount of design information in the Radiotron Handbook 4th edition. At the time the S-20R was designed it was a good choice for an all band receiver. The 6SA7 used in the S-40A is a superior tube but was not available when the S-20R was designed. toggle quoted message Show quoted text On 3/6/2025 3:36 PM, don Root wrote: An aside from ongoing topic ??S-20R mixer cct. ?… While trying to “see” how the LO circuit works? things got me in a big knot. C38 seems to run from some osc tanks back to the mixer RF tank area. When I follow that I get to? the common [not ground] of the coils/ transformers RF tanks? and found them to be floating = strange to me. I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting” -- don??? va3drl -- Richard Knoppow Los Angeles WB6KBL SKCC 19998
don Root All Messages By This Member #31946 Edited Thanks Tom for New schematic? .. did he just do it?? C38 does not seem to be connected as in Richards version.. see below please verify one way or the other. toggle quoted message Show quoted text From: [email protected] [mailto:[email protected]] On Behalf Of Allthumbs via groups.io Sent: Thursday, March 6, 2025 7:57 PM To: [email protected]; don Root Subject: Re: [HallicraftersRadios] S-20R circuit "analyisis" difficulties ? Hi all, ? Attached is a pdf of a crystal-clear re-drafting of a S-20R schematic. Perhaps it will be of use to anyone working on one of these sets. ? All credit to the author whose username on the Antique Radio Forum is bcascisa ? Tom On 03/06/2025 3:36 PM PST don Root <drootofallevil@...> wrote: ? ? An aside from ongoing topic ??S-20R mixer cct. ?… While trying to “see” how the LO circuit works? things got me in a big knot. C38 seems to run from some osc tanks back to the mixer RF tank area. When I follow that I get to? the common [not ground] of the coils/transformers RF tanks? and found them to be floating = strange to me. I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] ? IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting” ? -- don??? va3drl ? -- don??? va3drl image002.jpg
don Root All Messages By This Member #31947 开云体育 Richard, yours in black from previous, my adder in red Not sure I understand your question so feel free to disregard this if its nonsense. T-10, 11, 12, 13 are the plate tank of the RF amplifier stage. The common connections go to B+ the other side go to one side of the the band switch. ? The secondary coils go to the other side of the bandswitch which then goes to the signal grid of the mixer tube. ?Yes , and the secondary coils? seem to be tied together at the bottom? ..but ?otherwise floating so how does it couples to the grid unless inherent capacitance holds the common bottom to ground. I was hoping to keep this topic about the circuit ?and not the Problems in Tom’s radio. ???? -- don??? va3drl

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